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RADIUS packet format is shown below in the figure.
Code: This is 1 Octet long and identifies various types of packets. Normally 1 Octet means 1 Byte.
Identifier: This is again 1 Octet long and aids in matching responses with requests.
Length: This is 2 Octet long and specify the length of the packet including code, identifier, length and authenticator. ( Min packet is 20 Octet and max is 4096 Octet).
Authenticator: This is 16 Octet long and Filled up in case of some request and responses.
List of Attributes: There will be a list of 63+ attributes and a RADIUS attribute will also have a defined format which is described in next chapter.